Generally, a thermocouple is a device made from at least two different wires (thermoelements) that are joined at one end, hereinafter called the “hot” or “measuring” end. The other end of the thermocouple is referred to herein as the “cold” or “reference” end. To measure the temperature T2 of an environment, the measuring end may be exposed to the environment while the reference end is maintained at a temperature T1 (e.g., room temperature), wherein T1 is different than T2. Due to the difference in T1 and T2, a voltage difference can be measured between the two thermoelements using a voltmeter or other instrumentation. In this way, a thermocouple acts as a temperature-voltage transducer. This concept is shown in FIG. 1, which depicts a prior art thermocouple measurement system, in which voltmeter 105 is used to measure the voltage difference between first and second thermoelements 103, 104 when measurement end 101 is exposed to an environment having temperature T2 while reference end 102 is maintained at temperature T1.
The measured voltage can then be mathematically related to temperature at the measuring end using equation 1 below:
                    Emf        =                ⁢                              ∫                          T              1                                      T              2                                ⁢                                                    S                12                            ⁢                                                          ·              d                        ⁢                                                  ⁢            T                                                  =                ⁢                              ∫                          T              1                                      T              2                                ⁢                                                    (                                                      S                    1                                    -                                      S                    2                                                  )                            ·                                                          ⁢              d                        ⁢                                                  ⁢            T                              where EMF is the electro-motive force (voltage) produced by the thermocouple at the reference end, T1 and T2 are the temperatures as the reference and measuring ends, respectively, S12 is the Seebeck coefficient of the thermocouple, and S1 and S2 are the Seebeck coefficients of first thermoelement 103 and second thermoelement 104, respectively. A cold junction compensator (“CJC”) (not shown in FIG. 1) may also be employed to account for error that may be introduced due to differences in the composition of the voltmeter electrodes and thermoelements 103, 104. As may be appreciated, voltmeter 105 and a CJC may be incorporated into a thermocouple controller (also not shown), which may include other components such as a processor and memory.
Thermocouple performance may be enhanced when each of thermoelements 103, 104 is formed from a single wire that extends from measurement end 101 to reference end 102. For cost and other reasons, extension wires may be used to extend the length of thermoelements 103, 104. Such extension wires may facilitate connection of the thermoelements to a thermocouple controller, as described above. However variation in the composition of the extension wires and thermoelements 103, 104 may introduce unwanted voltages and temperature gradients, leading to erroneous temperature measurements. The cost of the extension wires may also be greater than the cost of copper wires. It may therefore be impractical and costly to produce thermocouples with integrated connectors, e.g., for automotive or other applications. This may limit the use of thermocouples in several applications, including automotive applications.
A type N thermocouple is a popular thermocouple type in the automotive industry. Although it offers reasonable accuracy, it cannot match the performance of type R and type S thermocouples, which include precious metal thermoelements. Since the thermoelements and extension wires of these prior art thermocouples must be made from the same alloy, cost often prohibits the use of precious metal type thermocouples in certain applications, such as automotive applications.
The need to use extension wires that are the same material as the thermoelements of a thermocouple may also limit the use of previous metal thermocouples in automotive applications. By way of example, an engine control unit (ECU) may include components that render it suitable for use as a thermocouple controller. However, such use may be precluded because the ECU connectors may be unsuitable for use with the extension wires and/or thermoelements of existing thermocouples. Moreover, in automotive applications it may be desirable to connect several thermocouples together to a single thermocouple controller. If one of the thermocouples connected to the controller fails, it may be necessary to replace the entire component (i.e., the control unit and all thermocouples connected thereto), thus increasing the cost of warranty service.